1. Field of the Invention
The present invention relates to a method and apparatus for processing microelectronic wafers. More precisely, the present invention relates to a method and apparatus for eliminating radio frequency arcing in a wafer coating plasma process.
2. Description of the Prior Art and Related Information
Semiconductor wafers containing microcircuits and dice are often coated with various types of film for specific desired purposes. Plasma Enhanced Chemical Vapor Deposition (PECVD) is such a process. It involves coating a wafer with a solid film of chemical substances generated from gaseous reactants through what is termed in the art as a radio frequency glow discharge. The PECVD coating process is carried out by suspending the wafers in a boat assembly that is, in turn, enclosed in a furnace tube (also called a quartz process tube) for processing. The boat assembly comprises a plurality of graphite plates, also known as paddles, arranged in parallel planes and separated by ceramic rods passing through the planes at about a right angle thereto. Wafer support pins are located on the graphite paddles and the wafers to be coated are suspended on the wafer support pins.
Graphite paddles are arranged on each ceramic shaft in a spaced apart relationship. Conventional ceramic spacers are mounted coaxially on the ceramic rod to separate adjacent graphite paddles from each other. Importantly, the spacers have a constant diameter for the entire length thereof, and do not pass through the graphite paddles.
As mentioned above, the boat assembly adopted for carrying the wafers is centered within the furnace or quartz process tube. The tube is enclosed in a resistance type heating element. Next, the chamber holding the boat assembly is evacuated to operating pressure and reacting gases are injected through the load/unload end part of the chamber. Vacuum integrity is maintained by use of a closed loop vacuum system that constantly monitors internal tube pressure. A radio frequency generator applies radio frequency energy to the boat assembly. Specifically, radio frequency power is applied to alternate plates in the boat assembly, thereby producing a plasma field. The RF power is applied only when the process variables such as pressure, temperature, and gas flow are stabilized. Usually, the PECVD process is carried out at low wafer processing temperatures from 40 to 650 degrees Celsius, due to the reaction being sustained by the plasma. The wafer is heated to assure film adhesion and desired characteristics. Reactions are typically carried out at pressures of 0.5 to 2 Torr.
Generally, all process functions are continuously monitored and controlled by a microprocessor. The microprocessor also monitors all safety interlocks and brings the process to a safe condition should an interruption or malfunction occur. As is commonly known in the art, the interlocks ensure that, for example, all seals on the furnace are tight, doors are secure, ambient pressures are equalized, etc.
A problem that exists with respect to conventional boat assemblies is that the structure is subject to "arcing" when RF energy is applied to the paddles. The arcing causes a short circuit during the process, which leads to defects in the deposited film. The cause of the arcing has been linked to graphite build-up on the ceramic rods that support the graphite paddles. During assembly and disassembly, when the paddles are slid over the ceramic shaft, the graphite buildup is scraped and graphite particles subsequently deposit on the outside diameter of the ceramic rods. The graphite acts as a conductor and when the RF energy is applied and the deposited graphite produces a short between the paddles. This arcing can result in increased manufacturing costs, loss of die on the silicon wafer, increased labor and downtime in the preparation of the boat assembly. Thus, a need presently exists for rectifying this arcing problem.